Current sensing is required for many types of circuit operations including current mode control, current monitoring, over-current protection and current dependent operation modes. Sensing current in a circuit typically involves the use of resistive elements. Sense resistors increase resistance and lower efficiency. An inductor can be used to sense the current instead of a sense resistor. In yet another alternative, the drain-to-source on resistance of a MOSFET (metal-oxide-semiconductor field effect transistor) can be measured to determine the amount of current flowing in the circuit.
Each of these conventional current sensing techniques require the current sensing signal generated by the sensing element to be routed back to a controller which manages one or more current related functions of the circuit. For example, when implementing current dependent operations in a driver stage of a power circuit, some means must be provided for sensing the current and communicating information about the sensed current back to the circuit. With discrete circuits, this typically involves providing additional feedback traces for connecting the sense device to the circuit and for feeding back the sensed current levels to the controller. All of these additional feedback traces must be carefully routed e.g. on a printed circuit board or within a multi-layer package to ensure normal circuit operation is not adversely affected by the current feedback mechanism, thus complicating the design of the board or package. Also, sensed current signals are usually very small in magnitude and can be rendered unreliable under certain noise conditions.